Alex Nail, a very talented landscape photographer from England, recently posted an interesting article about landscape photography manipulation. His article was prompted by an article he had read on 500px.com. Essentially, Alex feels that image manipulation including brightness, saturation and contrast adjustments are acceptable, but when one physically alters the landscape by, for example, adding a different sky to an image, one has gone too far. Ignacio Palacios, the landscape photographer to whom Alex refers, is much more accepting of physical alterations to the landscape, including creating composites, changing the shapes of mountains in Photoshop and so on.
My own take on this debate is that changes to brightness, contrast and saturation are wholly acceptable. In fact, a camera can't capture an image as the human eye sees it. Some photographers proudly declare "I don't edit my images." However, taking this approach is somewhat of a false economy as, even when you shoot in digital RAW format, some other entity such as Adobe or Canon chooses how to display the pixels your camera sensor captured on screen. By taking control of brightness, saturation and contrast, you as the landscape photographer are choosing how that sensor data is displayed.
When one starts adding other skies to a landscape photograph, adding trees, stretching mountains to make them taller, I feel that the transition from landscape photographer to digital artist has taken place. Several images of average quality could be combined by a skilled digital artist into an interesting landscape image. However, in my opinion this requires far less skill than the pure landscape photographer who takes the time to compose the image carefully, wait for the amazing light and so on. I class myself as a landscape photographer, not a digital artist and I am against compositing and making physical changes. My own limit would be removing a bird that had flown into frame or removing a piece of rubbish that had been left in the landscape (assuming I couldn't remove it in the field). If you look at the example below you will see a before and after of my typical landscape photography post-processing. I'm sure you will agree that I did all the hard work with the camera, not the computer.